Revisiting side-chain alkylation of toluene to styrene: Critical role of microporous structures in catalysts

Abstract

Due to potential advantages such as large availability and low cost of reactant feedstocks, the side-chain alkylation of toluene with methanol has been investigated as an alternative pathway for producing styrene. In this study, the effects of acid-base properties and microporous structures of zeolite catalysts were comprehensively investigated. The results showed that the main active sites for this reaction are base sites, which are required for activating the methyl group in toluene and converting methanol into HCHO, an alkylating agent. On the other hand, the role of acid sites is to stabilize toluene via Lewis acid-base interactions. Our results indicated that a significant dilemma exists in the design of the catalyst. Catalysts having sufficient basicity generally have limited Lewis acidity, which is ineffective for stabilizing toluene under the reaction conditions at elevated temperatures. Fortunately, the micropores of zeolites (e.g., X zeolites) provide secondary interactions such as van der Waals forces to toluene, which substantially enhance toluene stabilization, and thereby, the alkylation activity. In contrast, the model catalysts synthesized using mesoporous A zeolite showed no toluene adsorption at all due to the absence of any accessible microporosity. Consequently, even though the A zeolite catalysts have similar acid-base properties to those of the X zeolite catalysts, they showed no detectable toluene alkylation activity and produced only the decomposition products of methanol. The results clearly showed the importance of secondary interactions (or solvation effects) in zeolite catalysis, and explained why basic zeolites are superior to the conventional basic metal oxides that lack microporosity. (C) 2019 Elsevier Inc. All rights reserved.